Two methods of imparting hydrophobic character to textiles have been investigated in the past: 1) hydrophobic polymer films, and 2) attachment of hydrophobic monomers and polymers via physi- or chemisorptive processes.
Current commercial processes for producing water-repellent/soil-resistant fabrics are mainly based on the laminating processes of companies such as W. L. Gore and Sympatex (Journal of Coated Fabrics vol. 26, 1996, pp. 107-130) and polysiloxane coatings (Handbook of Fiber Science and Technology, Marcel Dekker, New York, N.Y., Vol. II, 1984, pp. 168-171). The laminating process involves adhering a layer of polymeric material (such as Teflon™ that has been stretched to produce micropores) to a fabric. Although this process produces durable repellent films, it suffers from many disadvantages. The application of these laminants requires special equipment and therefore cannot be applied using existing textile production processes. Synthesis of the film is costly and garments with this modification are significantly more expensive than their unmodified counterparts. The colors and shades of this clothing are limited by the coating color. Finally, clothing made from this material tends to be heavy and stiff. Polysiloxane films suffer from low durability to laundering, which tends to swell the fabric and rupture the silicone film. The polysiloxanes have a cost advantage over the laminates, which are, however, more durable to laundering and dry-cleaning.
Repellents based on monomeric hydrocarbon hydrophobes can be broken down into five categories: 1) aluminum and zirconium soaps, 2) waxes and waxlike substances, 3) metal complexes, 4) pyridinium compounds, 5) methylol compounds, and 6) other fiber-reactive water repellents. Compared to polymeric coatings, monomeric hydrophobes can penetrate within the fabric to produce a more durable coating.
The oldest and most economical way to make fabric water repellent is to coat it with a hydrophobic substance, such as paraffin (Text. Inst. Ind. vol.4, 1966, p. 255). This process is still in practice today and paraffin emulsions for coating fabrics can be purchased (e.g., Freepel® from BFGoodrich Textile Chemicals, Inc.). Waxes are not stable to laundering or dry cleaning. Durability is poor due to their noncovalent nature of binding and their breathability is low.
One of the oldest water repellents was based on non-covalently applying water-soluble soap to the fibers and precipitating it with an aluminum salt (J. Text Res. vol. 42, 1951, p. 691). These coatings dissolved in alkaline detergent solution, therefore washfastness was poor. Zirconium soaps were less soluble in detergent solutions (Waterproofing and Water-Repellency, Elsevier Publ. Co., Amsterdam, 1963, p. 188), but due to the noncovalent nature of attachment to the fabric, abrasion resistance was poor.
Quilon chrome complexes polymerize to form —Cr—O—Cr— linkages (Tappi vol. 36, 1953, p. 107). Simultaneously, the complex forms covalent bonds with the surface of fibers with hydrophobic chains directed away from the surface to produce a water repellent, semi-durable coating. Quilon solutions require acidic conditions to react, thus causing degradation of the cellulose fibers through cellulose hydrolysis. Fabric colors are limited by the blue-green coloration imparted by the metal complex.
The extensive history of pyridinium-type water repellents has been reviewed by Harding (J. Text. Res. vol. 42, 1951, p. 691). In essence, an alkyl quaternary ammonium compound is reacted with cellulose at elevated temperatures to form a durable water-repellent finish on cotton (Br. Pat. 466,817) and a later version was marketed under the trademark Velan PF by ICI. It was later found that the reaction was restricted to the surface of the fibers (J. Soc. Dyers Colour. vol. 63, 1947, p. 260) and the high cure temperature weakened the fabric. Sodium acetate had to be added to prevent the decomposition of the cellulose by the HCl formed. Also, the pyridine liberated during the reaction has an unpleasant odor and the fabric had to be scoured after the cure. The toxicological properties of pyridine ended its use in the 1970s when government regulations on such substances increased.
Methylol chemistry has been extensively commercialized in the crosslinking of cellulose for durable press fabrics. N-methylol compounds are prepared by reaction of an amine or amide with formaldehyde. Alkyl-N-methylol compounds can be reacted at elevated temperatures in the presence of an acidic catalyst with the hydroxyl groups of textiles to impart durable hydrophobic qualities (Br. Pats. 463,300 and 679,811). The reaction is accompanied by formation of non-covalently linked (i.e., non-durable) resinous material, thus decreasing efficiency. In addition, the high temperature and acid catalyst reduce the strength of the fabric. Recently, the commercial use of methylol compounds has been waning due to concerns of toxic formaldehyde release from fabrics treated in such a manner.
Several other chemical reactions have been used to covalently attach hydrophobic species to cotton to produce a water-repellent finish but have not been commercialized for various reasons. Long-chain isocyanates have been used in this respect (Br. Pat. 461,179; Am. Dyest. Rep. vol. 43, 1954, p. 453; Br. Pat. 474,403). The high toxicity of isocyanates and significant side reactions with water, however, precluded it from commercial use. To circumvent the water sensitivity of isocyanates, alkyl isocyanates were reacted with ethylenimine to yield the less reactive aziridinyl compound, which was subsequently reacted with cellulose at 150° C. (Ger. Pat. 731,667; Br. Pat. 795,380). Although the toxicity of the aziridinyl compound was reduced compared to the isocyanate, the procedure still required the handling of toxic isocyanate precursors. Also, the high cure temperature weakened the cellulose, and crosslinkers were needed to increase structural stability. Alkyl epoxides can be reacted with cellulose under acidic or basic conditions to produce durable, water-repellent cotton (Ger. Pat. 874,289). The epoxide was applied from a volatile solvent to suppress side reactions with water. Epoxides are, in general, not very reactive, thus requiring long reaction times at high temperatures. Therefore, they have not been commercialized. Acylation of cotton with isopropenyl stearate from an acidic solution of benzene and curing at 200° C. produced a durable hydrophobic coating (U.S. Pat. No. 4,152,115). The high cure temperature and acid catalyst again weakened the cotton. Carcinogenic benzene can be replaced by toluene, but the practicality of using flammable solvents in fabric finishing is limited. Alkyl vinyl sulfones react with cellulose in the presence of alkali to form a repellent finish (U.S. Pat. No. 2,670,265). However, this method has not been commercialized because the alkali is not compatible with cross-linking reactants required for permanent press treatments.
Conventional softeners improve the hand of the fabric as well as increase abrasion resistance and tear strength. The softener also functions as a sewing lubricant. There are four basic types of softeners—anionic, cationic, nonionic, and blended systems.
The anionic softeners are generally sulfated or sulfonated compounds used primarily to lubricate yarns through processing. Examples of these compounds include sulfonated tallow, glycerides, and esters. Sulfonated or sulfated castor oil, propyl oleate, butyl oleate, and tallow are used in various steps in dying fabrics. Anionics tend to provide inferior softness compared to the cationics and nonionics. Furthermore, they have limited durability to laundering or dry-cleaning. Their major limitation comes from their negative charge, which causes incompatibility in resin finishing baths and makes them most sensitive to water hardness and electrolytes.
The cationic softeners are nitrogen-containing compounds including fatty amino amides, imidazolines, amino polysiloxanes, and quaternaries. As a result of their positive charge, they are attracted to cotton or synthetic fabrics through electrostatic interactions. They tend to be compatible with most resin finishes and are somewhat durable to laundering. The most significant disadvantage of cationic softeners is their tendency to change the shade or affect the fastness of certain dyestuffs. Discoloration on white fabrics may also be a concern. The development of a fishy odor on the fabric can be a problem with certain systems.
Nonionics are the most widely used softeners. This class includes polyethylenes, glycerides such as glycerol monostearate, ethoxylates such as ethoxylated castor wax, coconut oil, corn oil, etc., and ethoxylated fatty alcohol and acids. The nonionic softeners offer excellent compatibility in resin baths due to their unreactivity. Since nonionics have no charge, they have no specific affinity for fabrics and therefore have relatively low durability to washing.
To optimize softening and lubricating properties, many manufacturers tend to formulate a softener containing both nonionic and cationic types. Typically, an aminosilicone or an imidazoline for a silky soft slick hand will be blended with a cationic or nonionic polyethylene lubricant for sewability and tear- and abrasion-strength properties. Increased customer demand for improved durability and useful life of a garment has led to the use of high-density polyethylenes as softeners. These have decreased solubility and thus are more durable. However, the disadvantages of the softeners (such as, for example, lack of durability to repeated launderings) remain.
The benefits of using the permanent modifying agent described below include durability of the treatment by providing covalent chemical attachment to the substrate. Additionally, the chemical nature of the modifier is compatible with other treatment formulations including, for example, water- or oil-repellent finishes and wrinkle-resistant treatments.
References: Handbook of Fiber Finish Technology, Philip E. Slade, Marcel Dekker, Inc., New York, 1998; Wellington Sears Handbook of Industrial Textiles, Sabit Adanur, Technomic Publishing, 1995, Pennsylvania; Cotton Dyeing and Finishing: A Technical Guide, Cotton Incorporated, North Carolina, 1996.